The present invention relates to a method for making an electrophotographic lithographic printing plate by reversal development, according to which a printing plate can be obtained by directly writing an image information output from electronic editing system and the like on an organic photoreceptor and carrying out reversal development.
Many methods have been known for making lithographic printing plates. For example, an original prepared by silver salt photography is allowed to contact with a light-sensitive material for printing plate and the light-sensitive layer of the material is directly exposed to ultraviolet rays thereby to form a hardened area corresponding to the image of the original and an unhardened portion corresponding to non-image. The unhardened portion is washed off with alkalis or water and the hardened portion is used as ink-receptive area. The printing plate made by this method is so-called PS plate and is widely used.
Recently, with progress of computer image processing techniques, and techniques for development of memory of high capacity data and data transmission techniques, electronic editing systems in which input, correction, editing, layout and composing of originals are all performed by computer controlling and the original can be immediately output to terminal plotters in a remote place by high speed communication networks or satelite communication have been put to practical use. Especially, in the fields of newspaper printing, not only the electronic editing systems, but also plate making systems by which printing plates can be directly obtained by the data coming from the electronic editing systems are being put to practical use. At present, in such plate making systems, those which carry out exposure to laser beam sources (such as semiconductor laser, He-Ne laser, etc.) utilizing electrophotographic process are expected from various aspects.
Furthermore, in such systems, electrophotographic lithographic printing plates of washing-off type mentioned below can be used for printing under the same printing conditions as for the conventionally employed PS plates and hence, the systems can be advantageously used. The main plate making steps are as follows: Electrophotographic material for printing plate is charged, exposed and developed to form images with toners and the non-image portion on which toners are not deposited is removed by dissolution solution to expose the surface of the support which has been previously subjected to hydrophilization treatment. Since the image portion protected with toners is oleophilic, offset printing can be performed.
An electrophotographic lithographic printing plate is made from an electrophotographic photoreceptor comprising an electrically conductive support such as aluminum and a photoconductor layer containing a photoconductive substance provided on the support. As materials which constitute the photoconductor layer of this electrophotographic photoreceptor, organic photoconductive compounds.cndot.binder resin systems as described in the following patent publications are excellent in practical sensitivity and printing endurance. Japanese Patent Kokoku Nos. 37-17162, 38-7758 and 46-39405 and Japanese Patent Kokai Nos. 52-2437, 57-161863, 58-2854, 58-28760, 58-118658, 59-12452, 59-49555, 62-217256, 63-226668 and 1-261659.
In the steps of making electrophotographic lithographic printing plates, the photoconductive layer is first subjected to a desired charging to apply uniform charge thereto and then a static latent image corresponding to the image is formed by exposing. This static latent image is developed with an electrophotographic developer and is fixed to produce a toner image corresponding to the static latent image. The non-image area other than the toner image is dissolved and removed (decoated) with a solution containing an alkali agent or the like and successively, the surface of the plate is subjected to treatment for adjustment of apparent pH with water or an acidic rinsing solution and, if necessary, coated with a surface protective solution (protective gum solution) to obtain a final printing plate.
If in the above developing step the toner image is to be obtained by so-called reversal development, good image free from fog and thinning in lines can be obtained owing to the development characteristics. Recently, image reproducibility with high resolution and high quality is demanded and the method for making electrophotographic lithographic printing plates by reversal development meets the demand for quality.
In carrying out the reversal development, first the portion to which toners are to adhere (line image portion) is irradiated with light and the potential of the light irradiated portion is decayed to nearly zero V in the exposing step. In this case, the portion which is not irradiated with light (the portion corresponding to non-image portion) retains the initial charge potential as it is. A given developing bias is applied in the developing step region through electrodes and a given developing electric field is formed by this developing bias. On the other hand, toner particles in the developer have a charge of the same polarity as in the photoreceptor side charged by a given method. Under such state, when the photoreceptor having the static latent image enters into the developing electric field of the developing step region, the toner particles in the developer are migrated to the photoreceptor side by the developing electric field and the toner particles deposit on the portion of 0 V in potential of the photoreceptor which is the portion irradiated with light. In this case, the non-image portion in which the initial potential remains repels the toner particles and toner particles do not deposit on this portion.
Either of a dry developer or a liquid developer can be employed as the developer used for such reversal development, but in the case of the liquid developer, toner particles can be made fine and hence, good image reproducibility with high resolution can be obtained.
The liquid developer is prepared by dispersing pigment (dye) or polymer particles in a highly insulating medium to carry out coloration and adding a charge control agent thereto to impart a desired charge.
Amount of the deposited toner in the developing step, namely, image density is determined depending on the various image forming conditions and the charge quantity of the toner particles mentioned above is an extremely important condition for determination of image qualities such as image density. That is, as aforementioned, in the reversal development, a portion of 0 in potential is formed by partial exposure of the initially uniformly charged area of photoreceptor side and this portion of 0 in potential is filled with the charge of the toner particles and thus the toner particles are allowed to adhere to or deposit on this portion. Therefore, amount of the deposited toner particles varies depending on charge quantity possessed by individual particle and in general, with increase in charge quantity of individual toner particles, amount of the deposited toner particles decreases and image density reduces and with decrease in charge quantity of the individual toner particle, amount of the deposited toner particles increases and image density enhances.
On the other hand, the charge quantity of the toner particles not only varies the image density, but also causes the following phenomena to occur.
First, consideration will be given to the case where a non-image portion in which the initial potential remains as it is, is continuously fed into a developing zone. If the developing bias potential applied from opposing electrodes is lower than the charged potential in this case, the toner particles charged in the same polarity as of the charge potential of the non-image portion are subjected to repelling power of the non-image portion and are forced back to the opposing electrode side. Therefore, when the non-image portion is continuous, the toner particles are electrically deposited and accumulated on the surface of the electrodes or localization of the toner particles gradually becomes conspicuous.
On the other hand, sometimes an additional electric field occurs from the toner particle which are electrically deposited and accumulated on the opposing electrodes or which are in densely localized state. The electric field generated from the toner particles is added to the bias voltage as an additional electric field in addition to the inherent developing bias electric field. When this additional electric field increases, a certain amount of toner particles sometimes abruptly begins to migrate to the photoreceptor side at a certain timing. This phenomenon occurs irrespective of non-image portion or image portion, resulting in deposition of the toner particles in a large amount. The inventors named this phenomenon "toner avalanching phenomenon".
If the above-mentioned toner avalanching phenomenon occurs in the developing step of electrophotographic lithographic printing plates, toner particles deposit on the non-image portion other than the desired image portion and prevent decoating of the non-image portion. When printing is carried out using the electrophotographic lithographic printing plate in which this phenomenon has occurred, reduction of resolution of prints and stain of prints are brought about. Thus, the toner avalanching phenomenon conspicuously deteriorates the quality as printing plate.
Furthermore, developing speed in development with liquid developer, namely, deposition amount of toners (image density) is determined by various conditions for image formation, but concentration of toner particles which is solid concentration in the liquid developer is a very important condition in determination of image quality based on the developing speed and image density. As aforementioned, in reversal development, a portion of nearly 0 in potential is partially formed by exposure in the initially uniformly charged area and deposition of the developer is performed by filling up the portion of nearly 0 in potential with the charge of toner particles. Therefore, developing speed (deposition amount of toners) varies depending on concentration of toner particles and generally, when concentration of toner particles is low, developing speed lowers and deposition amount of toner particles (image density) decreases and when concentration of toner particles is high, developing speed increases and deposition amount of toner particles (image density) increases.
However, as mentioned in detail below, in such liquid developing step, image reproducibility of so-called line image and that of solid image are contrary to each other and there is the problem that it is difficult to obtain well-balanced images.
First, in the case of line image, low potential portions (nearly 0 in potential) of thin width are formed in the form of lines in the high potential area formed by initial charging and high potential portions are arranged on both sides of the low potential portions of thin width in such a manner that the former are placed between the latter. Thus, line of electric force of high density is formed in the whole of the linear low potential portions due to the high potential portions arranged on both sides of the linear low potential portions. This line of electric force of high density very strongly forms an electric field, which is considerably greater than the inherent developing electric field formed by developing bias. Therefore, deposition of toners in this developing step of line images occurs along the line of electric force of high density and thus, development by so-called edge effect is carried out. In general, owing to this edge effect, the line images are protuberantly formed at the surface.
Therefore, in order to obtain faithfully a highlight portion including the line images, it is preferred to reduce the solid concentration, namely, toner particle concentration in the liquid developer than usual and to lower the developing speed.
On the other hand, for formation of solid images, a low potential portion (nearly 0 in potential) of large area is formed in the high potential area formed by the initial charging. The high potential portion is present around the low potential portion of large area, but the edge effect by the high potential portion affects only the portion along the periphery and only the inherent developing field is formed in most of the low potential portion.
Accordingly, in the case of forming the solid image, developing action of the developing bias electric field is main and in order to obtain faithfully a dot shadow portion including the solid image, it is preferred to increase solid concentration, namely, toner particle concentration in the liquid developer than usual and to increase developing speed. However, if the toner particle concentration is too high, defect of gradation may occur in the dot shadow portion.
As explained above, developing condition for obtaining faithfully the highlight portion including line image (low toner concentration) and developing condition for obtaining faithfully the dot shadow portion including solid image (high toner concentration) are contrary to each other and when the highlight portion is to be faithfully obtained, black solid portion in the dot shadow portion cannot be satisfactorily reproduced and especially when black lines in the highlight portion are to be obtained, white portion in the dot shadow portion disappears. When the dot shadow portion is to be faithfully obtained, black lines in the highlight portion disappear.
In the case of the printing plate of the present invention, non-image portion other than the toner image portion formed by reversal development is decoated with an alkali solution and the exposed surface of a hydrophilized aluminum base is rendered water-receptive and the image portion is rendered ink-receptive. The interface between the toner image portion and the non-image portion has very important influences on properties of printing plate formed by the dissolution with alkali, for example, not only image sharpness, but also printing endurance. That is, unless the interface is sharply formed, the boundary formed by dissolution with alkali is not clear and sometimes whisker-like blot is generated from the interface when the printing plate is completed after decoating of the non-image portion or image edge becomes irregular and thus, good prints cannot be obtained.
Therefore, for the printing plate it is necessary to keep balance between the highlight portion and the shadow portion and it is necessary to obtain high image sharpness in reversal development.